Delays occur in packet switched networks when switching a packet from one node in the network to another node. These delays can vary very significantly and, among other things, are due to the degree of utilization in the network and in individual nodes, and also to the policy of different nodes in forwarding a packet on arrival.
It is desirable to be able to measure the delays, so as to enable a network to be trimmed or to ascertain which part of the network are subjected to greater or smaller delays, for instance. Delays are measured by time stamping a packet, i.e. by storing in the packet a value which represents the time at which the packet leaves the node. The packet is then sent to another node in the network, and this node reads the stored value and compares the said value with the value on the time stamping clock in the own node, i.e. so-called two-time measuring. This provides a value of the delay. In order for this method to function satisfactorily, the time stamping clocks in the two nodes must show precisely the same absolute time, i.e. they must be synchronized.
In another delay measuring process, a one-time measuring process, the receiving node need not itself calculate the delay, and solely sends the packet immediately back to the original node. The value stored in the packet is read in the original node and compared with the current value of the time stamping clock. This obviates the need to synchronize two time stamping clocks, since only one time stamping clock is used. The measurement value obtained shows the delay in switching a packet to and fro. The delay in one direction is obtained by dividing the value by two. This results in a very good value, when delays are symmetrical. Unfortunately, this is not often the case, since the packet can travel to and fro along two different paths and in parts of the network that have significantly different nodes. This is particularly pronounced in the case of ATM networks (Asynchronous Transfer Mode).
Examples of one-time and two-time measuring processes are CTD (Cell Transfer Delay) and CDV (Cell Delay Variation).
In an ATM network, a performance management function (PM) divides a number of packets into a block. A PM packet follows each block and mediates different property related magnitudes, which include the value of the time stamping clock of the source node. This value is used in the target node to measure delays. If a combination of a fast link, e.g. 622 MB/s, and a small block size, e.g. 128 packets, is used, this will result in a large number of PM packets to the target node. This places great demands on the process capacity of the target node. The number of connections may also contribute towards the requirement of processor capacity.
Synchronization of time stamping clocks is a well-known problem in this field. NTP (Network Time Protocol) used to synchronize time stamping on the Internet can be quoted as an example in this respect. NTP has an accuracy of about one millisecond and is thus inadequate for ATM, for instance.
It is known from U.S. Pat. No. 5,280,629, Technique for Measuring Channel Delay, to use GPS equipment (Global Positioning System) to synchronize time stamping clocks for the purpose of measuring time delays between two nodes in a network.
One problem with the use of GPS for synchronizing time stamping clocks in accordance with U.S. Pat. No. 5,280,629 is that synchronization is lost very quickly if the GPS signal disappears, e.g. because of atmospheric interference and disturbances.